Vegetable oil based dielectric coolant

ABSTRACT

A transformer is disclosed, including a tank housing a transformer core/coil assembly, a dielectric insulating fluid surrounding the core/coil assembly, the fluid comprising a vegetable oil, an antioxidant dissolved in the oil, and a low temperature additive blended into the oil, wherein the fluid defines a headspace above the fluid. The preferred embodiment includes an oxygen absorbing material contained in the tank and in contact with gases in the tank but isolated from contact with the dielectric fluid, the tank including an opening and a sealed plug in the opening, such that the oxygen absorbing material can be replaced through the opening upon removal of the sealed plug, and further includes a gas permeable polymer container sealed against the opening for supporting the oxygen absorbing material and an indicator in gas contact with the headspace and visible from outside the tank for indicating the presence of oxygen in the headspace.

This is a divisional of application(s) Ser. No. 08/576,372 filed on Dec.21, 1995, now abandoned.

TECHNICAL /FIELD OF THE INVENTION

The present invention relates generally to the field of dielectriccoolants, or insulating oils, for use in electrical distribution andpower equipment, including transformers. More particularly, the presentinvention relates to a vegetable oil based insulating liquid, and stillmore particularly, to a composition comprising one or more vegetableoils and at least one additive that increases the suitability of thevegetable oil for use as a dielectric coolant. The present inventionalso relates to modifications of equipment, such as transformer tanks,that can further enhance the suitability of the vegetable oil for use asa dielectric coolant.

BACKGROUND OF THE INVENTION

An insulating liquid for use in electrical distribution and powerequipment, including transformers, has two important functions. First,it acts as an electrical insulating medium and, second, it transportsheat generated in the equipment. For example, heat is transported fromthe windings and core of the transformer or connected circuits tocooling surfaces. In addition to possessing the dielectric strength andcooling capacity, the ideal insulating liquid should be environmentallycompatible and relatively nonflammable.

For over a century, mineral oils derived from crude petroleum have beenused extensively as insulating and cooling liquids in electricalequipment. However, as safety standards became more demanding for manyindoor and vault equipment installations, such oils were replaced to agreat extent by non-flammable liquids, such as askarel (polychlorinatedbiphenyl-PCB) fluids. Beginning in the 1930's, PCB's, which aregenerally considered to be nonflammable, were widely utilized asreplacements for mineral oils as insulating liquids in electricalequipment. Nonflammability is a required property for insulating oilsthat are used in equipment placed within or around building structures,as it is necessary to minimize the hazard of fire and explosion damagein the event of electrical faults within the equipment.

Eventually, it became recognized that PCB's are environmentallyhazardous liquids. As a result, the production and sale of PCB's andtheir use in new equipment was banned. For existing PCB-containingequipment, severe regulations were issued requiring PCB removal atcertain installations and severe restrictions for all otherinstallations. In addition, spill reporting, clean-up and disposalrequire compliance with very strict regulations outlined in U.S. EPArules published in various editions of the Federal Register.Furthermore, due to their relatively poor ability to suppress arcs andharmful arc-degradation by-products, PCB-based fluids were not appliedto immersed safety and operational devices such as submerged powerfuses, circuit breakers, and load-break switches.

Because of the disadvantages and shortcomings of the polychlorinatedbiphenyls, there have been numerous efforts made to develop relativelyinexpensive, environmentally safe, nonflammable insulating oils. To datethese efforts have not been completely successful. It is the generalobject of the present invention to provide electrical equipmentutilizing an insulating liquid that is non-toxic, biodegradable,relatively nonflammable, innocuous to the environment, and comparativelyinexpensive. In addition, the insulating oils typically conform toexisting specifications or guides for dielectric fluids and must exhibitperformance characteristics that are generally comparable to presentlyused insulating oils.

Some of the functional properties of the oil and their significance areas follows. An oil's dielectric breakdown at 60 Hertz indicates itsability to resist electrical breakdown at power frequency and ismeasured as the minimum voltage required to cause arcing between twoelectrodes submerged in the oil. The impulse dielectric breakdownvoltage indicates its ability to resist electrical breakdown undertransient voltage stresses such as lightning and power surges. Thedissipation factor of an oil is a measure of the dielectric losses inthat oil. A low dissipation factor indicates low dielectric losses and alow concentration of soluble, polar contaminants. The gassing tendencyof an oil measures its tendency to evolve or absorb gas under conditionswhere partial discharge is present.

Because one function of the dielectric fluid is to carry heat, factorsthat significantly affect the relative ability of the fluid to functionas a dielectric coolant are viscosity, specific heat, thermalconductivity, and coefficient of expansion. The values of theseproperties, particularly in the range of operating temperatures for theequipment at full rating, are weighed in the selection of suitabledielectric fluids.

In addition to all of the foregoing properties that affect heattransfer, a dielectric fluid for commercial use should have a relativelyhigh dielectric strength, low dissipation factor, a dielectric constantcompatible with the solid dielectric, a low gassing tendency, and mustbe compatible with typical electrical equipment materials that areexposed to it. In order to function properly, the material must have anadequate heat transfer capability, which depends on its viscosity,specific heat and coefficient of expansion.

Current codes and standards further require that any dielectric fluidintended for use as a coolant must not be classified as Flammable, butrather as a Class IIIB Combustible liquid. The safety requirementsdepend on the application in which the electrical equipment containingthe fluid will be used, such as indoor, rooftop, vault, and adjacent tobuilding installations. According to the degree of hazard, one or moresafeguards may be required. One recognized safeguard option is thesubstitution of conventional mineral oil with Less-flammable andNon-flammable liquids. Less-flammable liquids must have an open-cup firepoint equal or greater than 300° C.

As described above, several operable fluids are known and used inelectrical equipment. However, due to increasing awareness andsensitivity regarding environmental concerns, it has become desirable toprovide a dielectric fluid that has minimal effect on the environmentand degrades quickly and easily enough so that spills will notcontaminate the soil or the water table for any significant period oftime, nor represent a significant hazard prior to the naturalbiodegradation process. It is becoming more desirable to replacenon-renewable resources with renewable resources, particularly in thearea of petroleum based products. There is increased demand bypurchasers for all-natural products. Finally, more attention is beingplaced on the long-term effects of materials and their degradationby-products. All these environmental, health, and safety trends favorthe use of vegetable based dielectric coolants over those derived frompetroleum.

The oils derived from various plants, herein referred to as "vegetableoils," include many oils that have suitable dielectric properties whenthe oil is fresh and carefully processed. It is often the case, however,that such oils are particularly susceptible to polymerization whenexposed to free oxygen. The rate of polymerization is directly relatedto the temperature of the oils at the time of exposure to free oxygen.Exposure to oxygen activates unsaturated bonds, causing oxidativepolymerization of the oil, with potentially adverse effects on bothequipment in the fluid and on the properties of the fluid itself.

Many types of electrical power distribution equipment, includingtransformers, are low-maintenance equipment that may go many yearswithout inspection. The presently used mineral oils are significantlyless susceptible to degradation due to exposure to oxygen than vegetableoils and therefore typically pass the standard oxidation stabilitytests. Therefore, mineral oils are well suited to use in this type ofelectrical equipment due to their long operable life. Correspondingly,until now there has been no acceptable way to effectively reduce thelong-term effects of exposure of vegetable oils to oxygen, so vegetableoils have not been successfully used as dielectric coolants in modernelectrical equipment. It is therefore desired to provide a lowmaintenance vegetable oil based dielectric coolant that meets or exceedssafety standards and is environmentally innocuous.

These and other objects and advantages of the invention will appear fromthe following description.

SUMMARY OF THE INVENTION

The present invention includes an insulating coolant composition basedon one or more vegetable oils and including various additives thatincrease the functional properties of the oil. The present compositionpreferably has low viscosity, high dielectric strength, and a high firepoint and includes a low temperature additive, an antioxidant, and anantimicrobial agent. The composition is selected to be stable over longperiods of use in electrical distribution and power equipment, andtransformers in particular. Because the present composition isessentially a natural food product, it poses no environmental or healthsafety hazard.

The present invention further comprises an oxygen scavenging device forremoving oxygen from the headspace of the electrical equipment. Theoxygen scavenging device is preferably an amount of an oxygen absorbingcompound, such as iron oxide, that is enclosed in a container thatprevents the oxygen absorbing compound from directly contacting thedielectric coolant. The container is preferably constructed of a gaspermeable, moisture/liquid impermeable material, so that any oxygen thatmay be present in the tank headspace will ultimately pass through it andbe absorbed inside the container.

The present invention further includes means for reducing the leakage ofoxygen-containing air into the equipment housing. These means includemodifications to the tank itself and gaskets used in sealing the tank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, cutaway side view of a transformer tank of thepresent invention;

FIG. 2 is a cutaway side view of a housing in the transformer tank foran oxygen scavenging material; and

FIG. 3 is a cutaway side view of the transformer with the tank andhousing of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention allows the use of vegetable based oils asdielectric fluids in electrical distribution and power equipment,including transformers. Vegetable oils typically comprise mixedglycerides formed from the combination of a polyol such as glycerinhaving a number of hydroxyl groups that have been esterified with anequal number of fatty acid molecules. Many vegetable oils aretriglycerides, i.e. have three fatty acids chemically bonded to theglycerin. The generalized formula for a triglyceride is: ##STR1## whereR₁, R₂, R₃ may be the same or different with carbon chains from C₄ toC₂₂ and levels of unsaturation from 0 to 3. Differences in vegetableoils are caused by variations in the fatty acid molecules. There areseveral different fatty acids, including myristic, palmitic, stearic,oleic, linoleic, linolenic, arachidic, eicosenoic, behenic, erucic,palmitiolic, docosadienoic, lignoseric, tetracosenoic, margaric,margaroleic, gadoleic, caprylic, capric, lauric, pentadecanoic andhepadecanoic acids. The fatty acids and resulting vegetable oils canvary in their degree of saturation. The three fatty acids on atriglyceride molecule may be all the same or may comprise two or threedifferent fatty acids. While triglyceride composition varies fromspecies to species, and less so from strain to strain of a particularspecies, vegetable oil derived from a single strain will haveessentially the same fatty acid composition.

Every naturally occurring triglyceride has unique properties. Forexample, some of the triglycerides are more susceptible to oxidationthan others. According to the present invention, it is preferred to useoils having fatty acids that include at least one degree of unsaturation(at least one C═C double bond). This mitigates the effects of oxidationand helps reduce the evolution of hydrogen gas that might otherwiseoccur. It has been found that oils containing mono-unsaturates oxidizeless readily than other oils and are therefore somewhat preferred foruse in the present application. Suitable vegetable oils include: soya,sunflower, rapeseed (canola), cottonseed, olive, safflower, jojoba,lesquerella, and veronia. All have fire points above 300° C.

Oxidation Avoidance

When the triglycerides of which vegetable oils are comprised are exposedto oxygen, they react to activate unsaturated bonds, causing oxidatvepolymerization of the oil. Products of such a reaction are undesirablebecause they have chemical properties that are inferior to the originalvegetable oil. It has been found that long-term degradation of the oil'sproperties due to oxidation requires long-term exposure to oxygen. Thus,for example, even if an oil is saturated with oxygen prior to testing,it can survive accelerated life testing without adverse effects if it isprevented from contacting additional oxygen during the test.

Therefore, it is desirable to provide a means for reducing the exposureof the oil to oxygen. By eliminating oxygen in the headspace of theelectrical equipment and minimizing the amount of oxygen initiallydissolved in the vegetable oil, the rate of the oxidation reaction maybe greatly reduced as described below. However, due to the prolongedoperational life expectancy of some electrical equipment, which istypically in excess of twenty years, it is desirable to provide furthermeans for reducing the overall reaction rate. According to the presentinvention, this is accomplished in part by dissolving an oxygenscavenging chemical in the vegetable oil. Examples of suitableantioxidants include BHA (butylated hydroanisole), BHT (butylatedhydrotoluene), TBHQ (tertiary butylhydroquinone), THBP, (Tetra HydroButro Phenone), ascorbyl palmitate (rosemary oil), propyl gallate andalpha-, beta- or delta-tocopherol (vitamin E). Other suitableantioxidants will be known to those skilled in the art.

Low Temperature Additives

Another factor critical to the performance of dielectric coolants aretheir low temperature physical properties, including pour point values.Typically, vegetable oils do not have natural pour points low enough tobe suitable for standard electrical power distribution applications. Anaverage electrical power distribution application will require a coolanthaving a pour point below -20° C. According to the present invention,the vegetable oil-based coolant is modified so as to ensure that it willremain a flowable liquid even when the equipment is subjected tomoderate low temperatures (lower than -20° C.) during its off-cycle.Modification of the oil includes the addition of a pour point depressantfrom the group including polyvinyl acetate oligomers and polymers and/oracrylic oligomers and polymers.

It has further been found that certain blends of oils have a lower pourpoint than either of the component oils have alone. For example, it hasbeen found that a blend of 25 percent soya oil (I) with 75 percentrapeseed oil (II) has a pour point of -24° C., as compared with -15° C.and -16° C. for (I) and (II) respectively. Some other combinations thatexhibit similarly advantageous reductions in pour point include: 25%soybean oil+75% oleate modified oil, 50% soybean oil+50% oleate modifiedoil, 25% soybean oil+75% sunflower oil. The addition of 0.1 % to 0.3%sorbitan tristearate will also reduce the pour point of the oil. It willbe understood that the list of combinations set out herein is notexhaustive, but is intended to be illustrative of the nature of theinvention.

It has further been found that vegetable oils exhibit a low temperaturebehavior that is different from that of mineral oils. Specifically, if avegetable oil is cooled to a low temperature that is slightly above itspour point temperature, so that it is still pourable, it may becomesolid or gelled upon prolonged storage at that temperature. It has alsobeen found that the low temperature stability of the oil can be improvedby the addition of one or more pour point depressant additives, and bythe blending of two or more oils, as described above.

Antimicrobial Additives

It is further preferred to include in the vegetable oil a compound toinhibit the growth of microorganisms. Any suitable antimicrobialsubstance that is compatible with vegetable oil may be used. Forexample, it is known that phenolic antioxidants such as BHA have someactivity against bacteria, molds, viruses and protozoa, particularlywhen used with other antimicrobial substances such as potassium sorbate,sorbic acid or monoglycerides. Vitamin E, ascorbyl-6-decanoate and otherknown compounds are also suitable for use as antimicrobial agents in theoil.

Water Removal

Because of its negative effect on dielectric performance, the presenceof water, a polar contaminant, in the fluid is undesirable. Water in thefluid will increase the rate of breakdown of fatty acid esters in thevegetable oil base in proportion to the amount of water available forthe reaction. The most obvious indicator of such reactions is asignificant increase in the value of the neutralization number due tothe increased acidity of the fluid. This reaction will lead to theformation of polar contaminants (ASTM D974).

The problem is compounded by the wide temperature range over whichelectrical distribution equipment must operate. It is known that thedielectric breakdown characteristics and other dielectric properties ofmineral oils are directly related to the percent of saturation of waterpresent in the oil. As the saturation point is reached, dielectricstrength falls rapidly. The saturation point at room temperature fortypical mineral oils used for dielectric coolants is approximately 65ppm at room temperature, and over 500 ppm at nominal operatingtemperature, approx. 100° C. However, electrical distribution equipmentis typically required to be able to operate over a wide temperaturerange, resulting in constant increases and decreases in the watercontent temperature necessary to achieve saturation. Water that isdissolved or in vapor/liquid equilibrium at a high operating temperaturemay precipitate or condense when the oil is brought to a lowertemperature.

Standards typically require moisture removal from conventional mineraloils to below 35 ppm for use in new distribution equipment. The moistureremoval process uses either evaporation in a reduced pressure chamber,filtration, or both to a typical level of 15-25% saturation at roomtemperature (10-15 ppm) prior to filling the distribution equipment.

Vegetable oils, in contrast, have a much higher water saturation points,typically well over 500 ppm at room temperature. Therefore, acceptablemoisture levels for use in new distribution equipment can be much higherthan that of conventional oils in terms of parts per million. However,due to the additional negative influence of water in vegetable oilcausing fatty acid ester breakdown, the moisture removal process shouldstrive for moisture levels as a percent of saturation well below thedesired values of mineral oil. Five to 10% of the saturation level isthe recommended range for vegetable oil at the end of the moistureremoval process.

Solids Removal

It has also been found preferable to remove various waxy particulatesand other minute solid contaminants from the oil by means of filtration.An example of suitable filtration means is a filtration medium capableof removing particulate matter as small as five (5) microns.

Processing

Each vegetable base oil will be processed to remove excessive moistureto a level of less than ten percent (10%) of the saturation level, andto remove particulates, and other contaminants, in similar manner to thecurrent practice of treating conventional mineral dielectric base oils.The treated base oils are then blended to achieve the desiredcompositions. To these blends, additives are added to improve certainkey properties of the compound, including antioxidant(s), antimicrobialagent(s), and pour point depressant(s). Once the materials have beenuniformly blended, the product is preferably stored in sealed systems orcontainers for future use.

Equipment Filling

The dielectric coolant must be properly introduced into the electricalequipment tank. The preferred process for tank filling minimizes theexposure of the coolant to atmospheric oxygen, moisture, and othercontaminants that could adversely affect its key properties. Thepreferred filling process includes drying of the tank contents,evacuation and substitution of air with dry nitrogen gas, filling underpartial vacuum, and immediate sealing of the tank. If the electricaldevice requires a headspace between the dielectric fluid and the tankcover, after filling and sealing the tank, the gas in the headspaceshould be evacuated and substituted with an inert gas, such as drynitrogen, with a stable positive pressure of between 2 and 3 psig at 25°C.

Properties of the Present Oil

It has been found that most vegetable oils have an open-cup fire pointwell above the accepted minimum standard (300° C.) for both conventionaldielectric oil and less-flammable fluids. For example, soya oilstypically have fire points of approximately 350° C. According to thepresent invention, the preferred oils have viscosities between 2 and 15cSt at 100° C. and less than 110 cSt at 40° C. and heat capacities(specific heats) greater than 0.3 cal./gm/°C.

Long term stability is enhanced by selection of most favorable vegetableoil blends, processing, and the addition of antioxidant andantimicrobial agents. Stability is further enhanced by controlling theenvironment to which the composition is exposed, particularly,minimizing oxygen, moisture and contaminant ingress into the tank, andby providing means for removing or capturing oxygen that might leak intothe tank.

Low temperature properties are improved by using optimal vegetable oilblends and by using pour point depressant additives. Together, thesemethods can result in pour points below -20° C., which is low enough formost standard electrical equipment applications.

Elimination of Oxygen in the Tank Headspace

It is also desirable to eliminate oxygen that may be present in theheadspace of electrical equipment containing a vegetable oil baseddielectric fluid. There are different approaches to electrical equipmentdesign. One design that is not suitable for use of vegetable basedinsulating coolants is the conservator non-sealed-type.

More common in ANSI/IEEE standard electrical distribution and mediumpower equipment design is the use of a tank headspace to allow forexpansion and contraction of the tank contents. Even if the headspace ofthe equipment is purged of air and replaced with inert gases, it ispossible over the operating life for oxygen (air) to leak into theheadspace due to openings of the cover or accessories, slow migrationthrough gaskets, and operation of the pressure relief device. Ingress ofoxygen into the headspace will eventually contribute to the consumptionof the antioxidant additives in the fluid. Hence, it is desirable toeliminate oxygen that may leak into the headspace of the tank.

One method for reducing the ingress of oxygen is to weld any components,covers or access points that communicate with the headspace, as gasketsand other means for sealing such openings are all susceptible to leakageover time.

This can be accomplished by providing a dry oxygen scavenging compoundin the headspace. In order to prevent contact between the oxygenscavenging compound and the vegetable oil, it is preferred to containsuch compound in an oxygen-permeable, oil- and moisture-impermeablepolymer container. Examples of suitable containers include those made ofpolyolefins including high density polyethylene, polypropylene,polybutylene, or polymethylpentene, and co-polymers thereof. Theselected material must sufficiently permeable to oxygen and must be ableto maintain the desired characteristics both at the high operatingtemperatures and in the broad range of temperatures to which the tank isexposed. A preferred material is a polymer film, which can be made intoa pouch for containing the oxygen scavenging compound.

A preferred oxygen scavenging compound is sold under the name Ageless bythe Cryovac Division of W. R. Grace & Company, Duncan, S.C. 29334. Theprimary constituent of Ageless is iron oxide. Alternatively, the oxygenabsorbing agent may comprise a mixture of ferrous salts with anoxidation modifier and/or metallic sulfites and sulfates compounds.These compounds react with oxygen according to the following formulas:

    Fe→Fe.sup.+2 +2e.sup.- 1/2O.sub.2 +H.sub.2 O+2e.sup.- →2OH.sup.- Fe.sup.+2 +20H.sup.- →Fe(OH).sub.2 Fe(OH).sub.2 +1/2O.sub.2 +1/2H.sub.2 O→Fe(OH).sub.3

In the above reaction, water is also reacted, which is advantageous inthe present application, as water is a polar contaminate that canadversely affect the dielectric properties of the oil.

Alternatively, an oxygen removing compound can be provided according tothe teachings of U.S. Pat. No. 2,825,651, which discloses an oxygenremover comprising an intermixing of a sulfite salt with an acceleratorsuch as hydrated copper sulfate, stannous chloride, or cobaltous oxide.A second alternative oxygen scavenging compound is disclosed in U.S.Pat. No. 4,384,972, which teaches the use of a salt of manganese, iron,cobalt or nickel, an alkali compound and a sulfite or a deliquescentsubstance.

Examples of other compounds that can be used to scavenge oxygen from theheadspace include: a combination of carbon and activated iron powder,mixtures of hydrosulfite, calcium hydroxide, sodium bicarbonate andactivated carbon, a metal halide powder coated on the surface of a metalpowder, and combinations of an alkali compound such as calcium hydroxidewith sodium carbonate or sodium bicarbonate.

The following description is given in terms of an electricaltransformer. It will be understood by those skilled in the art that thecompositions and method set forth are equally suited to use in othertypes of electrical equipment, including, but not limited to: reactors,transformers, switchgear, regulators, tap changer compartments, highvoltage bushings, etc.

Referring now to FIG. 1, a transformer tank 10 typically comprises atank body 12, a tank cover 14 bolted or welded to tank body 12 andsealed with a gasket 16. Tank body 12 is sealed. Tank 10 houses thetransformer core and windings (not shown) or other electrical equipment,which are immersed in a dielectric insulating fluid 18. The spacebetween the surface of the fluid and the tank cover is the tankheadspace 20. According to one embodiment of the present invention, apolymer container 22 containing oxygen scavenging material is mounted inthe headspace of the tank, preferably on the inside of the tank cover asshown in FIG. 1. A set forth above, container 22 is preferably a pouchor bag constructed of gas-permeable film. According to a more preferredembodiment shown in FIG. 2, the container 22 is supported in apolyolefin housing 24 mounted adjacent to a threaded opening 26 in thetank cover. A threaded plug 28 seals the container in the opening in thetank cover 14 and preferably includes a transparent viewing port 30. Itwill be understood that view port 30 can alternatively be incorporatedinto another part of the tank cover or walls.

When it is desired or necessary to replace the container of oxygenscavenging material, the threaded plug 28 is removed, and the container22 is removed from the polyolefin housing 24 and replaced. The low gaspermeability of housing 24 prevents significant gas exchange between theheadspace 20 and the outside atmosphere during the short period that thethreaded plug is removed. This can be accomplished even though the gaspermeability of the container is not so high as to impede operation ofthe oxygen scavenging material over more extended periods of time.

Still referring to FIG. 2, in addition to the oxygen scavengingmaterial, it is preferred to provide a means for indicating the presenceof oxygen in the tank headspace. This indicator is preferably an oxygensensitive compound 32 such as that marketed by the Mitsubishi GasChemical Company and distributed in the United States by the CryovacDivision of W. R. Grace and Company under the trade name Ageless Eye.This compound exhibits a pink-to-blue color change when the ambientoxygen concentration exceeds 0.1%.

The oxygen indicator is preferably housed in the tank headspace wall insuch a manner that it can both chemically contact the gas in theheadspace and be visible for inspection from outside the tank. One wayto accomplish this is to mount the oxygen indicator adjacent to the viewport 30 as shown.

Tank Modifications

In addition to the foregoing, the use of vegetable oil based dielectricinsulating fluids in transformers is facilitated by severalmodifications to the transformer tank. These include providing thesealed, accessible chamber described above, in which the oxygenabsorbing material can be replaced without increasing the exposure ofoil in the tank to air. Other modifications reduce the leakage of airinto the tank, so as to reduce the long-term exposure of the oil to air.

Referring now to FIG. 3, one such modification relates to the volume ofthe tank headspace 20. For example, current ANSI/IEEE C57 seriesstandards require distribution transformer tanks to remain sealed over atemperature range of from -5° C. to +105° C. for pole and padmounteddesigns and from -20° C. to +105° C. for substation transformers.Outside this range the tank is typically vented to avoid damage to thetank or related equipment. According to the present invention, the headspace volume is increased so that the temperature range over which thetank remains sealed increases correspondingly, thus reducing, theprobability of oxygen (air) leaking into the tank. Specifically, thepresent tank preferably includes a head space volume sufficient to allowthe tank to remain sealed from -20° C. to +115° C.

In addition, each tank includes an automatic pressure release device(PRD) 40 for venting, the tank as described above. According to thepresent invention, the PRD 40 is calibrated to automatically vent headgas space only when the internal pressure exceeds 9±15 psig, and toautomatically reseal when the pressure reduces to 6±1 psig. Because thePRD reseals at a positive pressure, the head space will maintain apositive pressure even after venting, by the PRD. Maintaining, apositive pressure in the head space helps prevent the ingress of airinto the tank.

In addition to the foregoing, it is also preferred to replace theconventional gaskets (not shown) with gaskets made from a material thatis substantially gas impermeable. It will be understood that such gasketmaterial must also be resistant to degradation by the dielectriccoolant. Examples of a suitable gasket material include nitrile rubberwith a high acrylonitrile content, and various fluoroelastomers, ofwhich the compound sold under the name VITON, a trademark of the E.I. duPont de Nemours & Company, is representative. In contrast, siliconerubber, and nitrile rubber having a low acrylonitrile content arebelieved to be less suitable, due to relatively high gas permeability.It will be understood that this list is illustrative only, and thatother resilient, gas impermeable materials could be used to form thegaskets for the transformer tank. As mentioned above, another way toavoid the leakage associate with the long-term use of gaskets, is toweld the equipment housing shut, completely eliminating the gasketedseals.

Another method of reducing gas ingress is to eliminate the head space byproviding for thermal expansion by other means. The pressure/partialvacuum withstand would be based on a thermal range of the average fluidtemperature of -20 through 115° C.

For units with sufficient headspace, vegetable oil based dielectriccoolants could also serve as an excellent material in the recentdevelopment of High Temperature Transformers, which typically have amaximum top oil rated temperature rise over ambient of 115° C.

Internal Insulation Modification

In addition to the foregoing, vegetable oil based dielectric insulatingfluids in electrical equipment in which paper insulation has beensubstituted by non-cellulose insulating "paper" would have greaterinherent stability. This is due to the fact that cellulose materialsliberate water as they are thermally degraded. Candidate materialsinclude aramid insulating material, polyester materials, polamid, etc.

While a preferred embodiment of the invention has been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit of the invention.

What is claimed is:
 1. A transformer including a tank housing atransformer core/coil assembly, comprising:an essentially food gradedielectric insulating fluid surrounding said core/coil assembly, saidfluid comprising a vegetable oil and an antioxidant dissolved in saidoil; and an oxygen absorbing material contained in said tank and incontact with gases in said tank but isolated from contact with saiddielectric fluid.
 2. A transformer including a tank housing atransformer core/coil assembly, comprising:an essentially food gradedielectric insulating fluid surrounding said core/coil assembly, saidfluid comprising a vegetable oil, an antioxidant dissolved in said oil,and a low temperature additive blended into said oil, said fluiddefining a headspace above said fluid; and an oxygen absorbing materialcontained in said tank and in contact with gases in said headspace butisolated from contact with said dielectric fluid.
 3. The transformeraccording to claim 2 wherein said oxygen absorbing material comprises airon oxide.
 4. The transformer according to claim 2 wherein said oxygenabsorbing material is housed in a gas permeable, liquid impermeablecontainer.
 5. The transformer according to claim 4 wherein saidcontainer is a pouch constructed of a polymer film.
 6. The transformeraccording to claim 2, further including an opening in the tank and asealed plug in said opening, such that said oxygen absorbing materialcan be replaced through said opening.
 7. The transformer according toclaim 6, further including a view port for viewing said oxygen absorbingmaterial.
 8. The transformer according to claim 7 wherein said view portis incorporated in said sealed plug.
 9. The transformer according toclaim 6, further including a container sealed against said opening forsupporting said oxygen absorbing material such that when said plug isremoved said oxygen absorbing material can be replaced without asignificant gas exchange between said headspace and the atmosphere. 10.The transformer according to claim 6, further including an indicator ingas contact with said gases in said headspace for indicating thepresence of oxygen in said headspace.
 11. The transformer according toclaim 10 wherein said indicator undergoes a visible color change uponexposure to a predetermined level of oxygen.
 12. The transformeraccording to claim 10 wherein said indicator is mounted in saidheadspace so as to be visible from outside the tank without opening thetank.
 13. The transformer according to claim 2 wherein said oxygenabsorbing material comprises a combination of a sulfite salt with anaccelerator selected from the group consisting of hydrated coppersulfate, stannous chloride and cobaltous oxide.
 14. The transformeraccording to claim 2 wherein said oxygen absorbing material comprises acombination of carbon and activated iron powder.
 15. The transformeraccording to claim 2 wherein said oxygen absorbing material compriseshydrosulfite, calcium hydroxide, sodium bicarbonate and activatedcarbon.
 16. The transformer according to claim 2 wherein said oxygenabsorbing material is selected from the group consisting of a metalhalide powder coated on the surface of a metal powder; and a combinationof an alkali compound with a sodium carbonate.
 17. The transformeraccording to claim 2 wherein said core/coil assembly is furtherinsulated with layers of a non-cellulosic insulating material.
 18. Atransformer including a tank housing a transformer core/coil assembly,comprising:an essentially food grade dielectric insulating fluidsurrounding said core/coil assembly, said fluid comprising a vegetableoil, an antioxidant dissolved in said oil, and a low temperatureadditive blended into said oil, said fluid defining a headspace abovesaid fluid; an oxygen absorbing material contained in said tank and incontact with gases in said tank but isolated from contact with saiddielectric fluid; said tank including an opening and a sealed plug insaid opening, such that said oxygen absorbing material can be replacedthrough said opening upon removal of said sealed plug; a gas permeablepolymer container sealed against said opening for supporting said oxygenabsorbing material; and an indicator in contact with said gases in saidheadspace and visible from outside the tank for indicating a presence ofoxygen in said headspace.
 19. A method of using a transformer comprisingthe step of employing an essentially food grade dielectric fluidcomprising a vegetable oil having at least one degree of unsaturationand a fire point above 300° C.
 20. The method of claim 19 in which thevegetable oil is a flowable liquid at temperatures lower than -20° C.21. The method of claim 19 in which the vegetable oil comprises a blendof two or more vegetable oils wherein the dielectric fluid has a lowerpour point than each of the two or more oils.
 22. The method of claim19, further comprising an oxygen scavenging material in contact with thedielectric fluid.
 23. The method of claim 22 wherein the oxygenscavenging material is dissolved in the dielectric fluid.
 24. The methodof claim 22 wherein the oxygen scavenging material is contained withinan oxygen permeable, fluid impermeable membrane.
 25. A transformerincluding a tank housing a transformer core/coil assembly, comprising:anessentially food grade dielectric insulating fluid surrounding saidcore/coil assembly, said fluid comprising a vegetable oil; and means forreducing exposure of the vegetable oil to oxygen.
 26. The transformeraccording to claim 25, said exposure reducing means comprising anantioxidant dissolved in said oil.
 27. The transformer according toclaim 25, said exposure reducing means comprising an oxygen absorbingmaterial contained in said tank and in contact with gases in said tankbut isolated from contact with said dielectric fluid.